Treatment failure in filariasis mass treatment programmes.

The data from the first 3 years of the Samoan pilot filariasis control programme were reanalysed using incidence instead of prevalence statistics. With these statistics, failures of diethylcarbamazine treatment can be roughly divided into three groups: primary treatment failures, manifested by persistent microfilaraemia; secondary treatment failures, manifested by microfilaraemia recurring within the prepatent period after apparently successful treatment; and new infections. When cases of persistent and recurrent microfilaraemia are excluded by appropriate statistical manipulations, the incidence of new infections is seen to be many times higher in persons who originally showed microfilaraemia. This suggests that susceptibility or exposure, or both, are not homogeneously distributed in the population, and indicates that proposed mathematical models of filarial epidemiology must be further refined. It also indicates that filariasis control programmes should devote more attention to studies and control methods aimed at this high-risk subgroup of the population.